The genetics, etiology and pathogenesis of insulin-dependent diabetes mellitus (IDDM) is yet to be clarified to explain the specific loss of the pancreatic beta cells which is pathognomonic to this disease at the time of clinical diagnosis. The natural history of IDDM is associated with a number of immune abnormalities which precede diagnosis and include early appearance of islet cell antibodies followed by an infiltration of the islet cells by mononuclear cells including macrophages, T and B lymphocytes. Studies of idiopathic IDDM in man, the NOD mouse and the BB rat suggest that a gene or genes in the major histocompatibility complex (MHC) are necessary but not sufficient for the disease. Other genes are essential but have proved difficult to detect in man. The complex inheritance of human IDDM may be better understood by first studying genetically defined animals with spontaneous diabetes and then go from the NOD mouse and the BB rat to man in order to define homologous genes which control the development of diabetes. We propose to use the spontaneously diabetic BB rat which is a best model for human IDDM. Based on BB rats, inbred for more than 30 generations we have shown in crosses between specific pathogen-free diabetes prone (DP) and diabetes resistant (DR) lines, that diabetes always is inherited in the presence of lymphopenia suggesting that the two phenotypes may be controlled by a single gene. In collaboration with Howard Jacob and Eric Lander this gene (iddm1) was mapped to rat chromosome 4. Using crosses between DP and Fischer F344 or Lewis rats we will dissect out the genes responsible for IDDM. These genes will be cloned and sequenced to become candidate genes for human IDDM. In collaboration with Howard Jacob and Eric Lander we specifically propose: 1) to the dissect the gene (iddm3) which confer resistance to IDDM in a cross between DP and Fischer rats: Marker assisted breeding will be used to characterize the backcross rats and the gene mapped using the total genomic search strategy of simple sequence length polymorphism (SSLP) of the rat genetic linkage map; 2) to determine the structure and function of iddm1 and iddm3. We will prepare overlapping cosmid clones to identify DP/DR polymorphisms and search for recombinant F2(DRxDP) rats to locate iddm1 and iddm3 and perform positional cloning in collaboration with Howard Jacob to eventually identify, sequence and determine the function of these two genes; 3) to determine if corresponding human genes are linked to IDDM; We will determine regions of conservation between human and rat chromosomes and develop polymorphic genetic markers for man based on the BB rat research. These markers will be used to determine their co-segregation with IDDM in HBDI families using an affected sibpair analysis and in population-based IDDM children and controls to determine the relative risk in combination with HLA-DR/DQ to test the hypothesis that a combination of genetic and islet cell antibody markers increases the predictability of IDDM in the general population. These studies should identify genetic loci essential for insulin-dependent diabetes mellitus in both the BB rat and man.